Question

Problem 3

The block diagram of the architecture (internal organisation) of an 8088 microprocessor is shown below.

a.Explain the basic operation of the EU and BIU units.

b.Briefly explain the operation of the following:

1. General registers
2. Pointer and Index registers
3. Flags
4. Segment Registers
5. Instruction pointer
6. Σ block

Answer 1

A.

• The Execution Unit (EU):

The main components of the EU are the ALU, General purpose registers, Special purpose registers, Instruction Register and the Flag/Status Register and the Instruction Decoder. It helps to Fetche instructions from the Queue in BIU,and then decodes after that it executes arithmetic and logic operations using the help of ALU.
It also help in Sending control signals for internal data transfer operations within microprocessor.
In order to access the external module it request signals to the BIU .
It operates w.r.t T-clock cycles and not machine cycles.

• The Bus Interface Unit (BIU): It is used to provides the interface of 8086 to external memory and I/O devices.
  It performs various machine cycles to transfer data between memory and I/O devices.
  It is used to fetch instructions from the memory
  It helps to generate 20 bit physical address for the use of memory access.
  It also maintains the 6 byte re fetch instruction queue(which ultimately help to supports pipe lining).

• General registers:General registers are divided into two sets: data and address.
  the address registers and are used to point to the locations in memory where data will be retrieved or stored as it contain memory addresses The data registers are for calculations;

• Pointer Register to are used during instructions like CALL, RET,PUSH, POP, etc
  It can also hold offset address of any location in the stack segment. and are used to access random locations of the stack.

• Index register help to hold the offset address in Data Segment and extra datd during string operations.

• Flags are used to help change or recognize the state of the microprocessor.
  there are 9 types of flag :6 status flag and 3 Control flag
  The Control flags are used in order to control certain operations like CLC, STC, CLD, STD, etc and after every every arithmetic and logic operation Status flags are updated

• Segment help to hold the base address for the Code Segment.
  So programs are stored in the Code Segment and accessed via the IP. It also help to holds the base address for the Data Segment. base address for the Stack Segment as well as extra segment.

• Instruction Pointer (IP): Using 16 bits of segment register CS all program instructions located in memory are pointed so 16 bits offset contained in the 16 (IP). When BIU computes the 20 bit physical address internally the contents of CS and IP. 16 bit contents of CS will be shift 4 bits to the left and then adding the 16 bit contents of IP. Relative contents of IP are all program instructions located in memory are then pointed.It basically help in calculation the next instruction whenever a branch instruction occurs so IP gets a new value

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